Radiation detector

ABSTRACT

This invention provides a radiation detector using a scintillator having both a strong luminescence intensity and a short time constant.  
     This invention is a radiation detector comprising as a scintillator an organic/inorganic perovskite hybrid compound represented by the general formula AMX 3 , wherein A is R—NH 3  or R′—NH 2 , or a mixture thereof, R is a hydrogen atom or a methyl group which may be substituted by an amino group or a halogen atom, R′ is a methylene group which may be substituted by an amino group or a halogen atom, each X is a halogen atom that may be identical to or different from the other X groups, and M is a Group IVa metal, Eu, Cd, Cu, Fe, Mn or Pd.

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates to a radiation detector for ionizingradiations, such as γ-rays, X-rays, electron beams, heavy chargedparticle beams and neutron beams, and more specifically, having a veryshort time from a rise to a disappearance of luminescence ofsubnanosecond order.

PRIOR ART

[0002] A scintillator detects and measures ionizing radiation opticallyby using a solid radiation relaxation phenomenon. In recent years, infields such as physics, chemistry, biology and medicine, the use ofshort-pulsed radiation is becoming more widespread and simple methods ofmeasuring short-pulsed radiation are required. For example, in the caseof a PET (positron emission tomography) which is a medical device, thetime resolution of the scintillator is directly linked to the spatialresolving power of the system, so the higher the resolving power of thescintillator is, the more precise the diagnosis that can be performed.There is therefore a demand for a high resolving power scintillator.

[0003] Scintillators may use organic crystals such as anthracene,inorganic crystals such as sodium iodide doped with thallium, orceramics such as PWO which have recently been developed, but even thosewith a fast luminescence decay time constant are only of nanosecondorder. Among scintillators in practical use, barium fluoride is uniquein having a decay time constant (600 picoseconds) of subnanosecond order(M. Laval, M. Moszynski, R. Allemand, E. Cormoreche, P. Guinet, R. Odruand J. Vacher, Nucl. Instru. Meth., 206 (1983) 169), but as itsluminescence wavelength is in the ultraviolet region, there are severepractical restrictions to its use. Such inorganic scintillators canroughly be classified into two groups. The first group have a largeluminescence quantum efficiency but a slow time constant of 200nanoseconds or more (NaI(Tl), CsI(Tl), CsI(Na), BGO, CdWO₄), and theother group has a small luminescence quantum efficiency and a fast timeconstant of 1 to 30 nanoseconds (BaF₂, CsF, CeF₃, CsI, organicscintillators). For example, GSO(Ce) has an intermediate luminescenceintensity and an intermediate time constant (60 nanoseconds), but itsperformance does not satisfy practical requirements (Carel W. E vanEijk, “Nuclear Instruments & Method in Physics Research SectionA—Accelerators Spectrometers Setectors and Associated Equipment” Nucl.Instr. and Meth. A 392:(1-3)285-290 Jun. 21, 1997, 460:(1)1-14 Mar. 11,2001).

[0004] Thus, an ideal scintillator has not yet been discovered, but thesearch for a material having a high luminescence intensity and a shorttime constant is continuing.

PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] The Inventors already proposed a radiation detector using atwo-dimensional stratified compound (R—NH₃)₂MX₄ as an organic/inorganichybrid compound scintillator (Japanese Patent Application No.2001-006132). This compound has an exciton with a very large boundenergy in a self-organized quantum well structure. Its decay timeconstant is approx. 100 picoseconds, and of the scintillators which haveso far been reported, it is therefore one of the substances with theshortest time from rise to disappearance of luminescence. However, asits crystallizing ability is low compared with inorganic crystals orceramics, it is difficult to produce crystals having a large volume.Therefore, for detecting a high LET radiation pulse such as a heavycharged particle, sufficient scintillation efficiency can be obtainedeven with a spin coat film, but for detecting a low LET radiation pulsesuch as a γ-ray and high-speed electron beam, it has the disadvantagethat its scintillation efficiency falls as the LET (linear energytransfer) decreases.

MEANS TO SOLVE THE PROBLEMS

[0006] This invention solves the above-mentioned problem, and provides aradiation detector using a scintillator having a large luminescenceintensity and a short time constant.

[0007] Specifically, the Inventors discovered that whenthree-dimensional perovskite organic/inorganic hybrid compoundsrepresented by the general formula AMX₃ (wherein, A, M, X are asdescribed later) were excited by an ionizing radiation, intenseradiation accompanied the relaxation step, this scintillationluminescence had a single peak in the visible region, and although thetime from rise to disappearance of this luminescence was longer than inthe case of the two-dimensional stratified compound (R—NH₃)₂MX₄, it wasshorter than in the case of other ordinary scintillators.

[0008] Further, the Inventors succeeded in growing good quality, highvolume crystals of this three-dimensional compound from a solutionthereof, and discovered that in a test where it was irradiated with ashort pulse electron beam, the radiation relaxation step was ahigh-speed exciton luminescence of subnanosecond order.

[0009] As the three-dimensional compound AMX₃ (wherein, A, M, X are asdescribed later) does not have a multi-layer structure comprising aninorganic layer and organic layer as in the case of the two-dimensionalcompound (R—NH₃)₂MX₄, the exciton bound energy is low (in the formercase, approx. 40 meV, and in the latter case, approx. 300 meV), howeveran exciton luminescence having a peak wavelength of 550 nm at roomtemperature was still observed.

[0010] This three-dimensional compound forms a three-dimensional networkwherein clusters comprising six halogens X coordinated to a divalentmetal are shared, so compared with the two-dimensional stratifiedcompound (R—NH₃)₂MX₄, the crystallizing ability is high. Therefore,crystals of large volume can easily be obtained, and an improvement ofscintillation efficiency for low LET radiation can be realized.

[0011] In view of the characteristics of this three-dimensionalcompound, the compound, and in particular its crystals, can be widelyused as a high-speed response scintillator for ordinary ionizingradiations. On the other hand, the two-dimensional stratified compound(R—NH₃)₂MX₄ may be used for special cases as an ultra high-speedscintillator for very short super-single pulsed radiations, or as ascintillator for the simple detection of high LET radiations by a spincoat film of this compound. Therefore, these two compounds may be usedin different situations according to the application.

[0012] In particular, crystals of the three-dimensional compound of thisinvention allow the detection of low LET radiations such as γ-rays andX-rays, which was difficult using the two-dimensional stratifiedcompound of the prior art, and offer a higher time resolution than thatprovided by ordinary scintillators such as other inorganic crystals,organic crystals or ceramics.

[0013] Specifically, this invention is a radiation detector comprisingas a scintillator an organic/inorganic perovskite hybrid compoundrepresented by the general formula AMX₃ wherein A is R—NH₃ or R′—NH₂, ora mixture thereof, R is a hydrogen atom or a methyl group which may besubstituted by an amino group or a halogen atom, R′ is a methylene groupwhich may be substituted by an amino group or a halogen atom, each X isa halogen atom that may be identical to or different from the other Xgroups, and M is a Group IVa metal, Eu, Cd, Cu, Fe, Mn or Pd. An exampleof this perovskite organic/inorganic hybrid compound where A is amixture, is (CH₃NH₃)_((1-x))(NH₂CH═NH₂)_(x)PbBr₃ (0<x<1). This radiationdetector is suited to detect low LET radiation, and in particular thelow LET radiation is a pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 shows the basic structure of an organic/inorganicperovskite hybrid compound according to this invention.

[0015]FIG. 2 shows a schematic view of a device for manufacturing singlecrystals of the perovskite compound by the poor solvent diffusionmethod. A is a glass bottle into which the perovskite compound isintroduced, B is a glass bottle into which a poor solvent is introduced,and C is a desiccator.

[0016]FIG. 3 shows a schematic view of a device which excites singlecrystals of PbBr (CH₃NH₃)₃ using the electron beam pulse of a linearaccelerator (LINAC), and allows observation of the luminescence.

[0017]FIG. 4 shows the time profile of the scintillation of PbX (CH₃NH₃)3.

[0018]FIG. 5 shows the temperature dependency of the luminescenceintensity (ultraviolet irradiation) of single crystals of the perovskitecompound.

[0019]FIG. 6 shows a device showing that single crystals of theperovskite compound can detect γ-rays.

[0020]FIG. 7 shows the scintillation emission spectrum when singlecrystals of the perovskite compound are irradiated by γ-rays.

DETAILED DESCRIPTION OF THE INVENTION

[0021] In the scintillator of this invention represented by MX₃(wherein, A is R—NH₃ or R′═NH₂, or a mixture thereof), A is a monovalentcation with a small volume such as [CH₃NH₃]⁺ or [NH4]⁺. Thetwo-dimensional organic/inorganic perovskite hybrid compound (R—NH₃)₂MX₄of the prior art uses an alkyl group (C_(n)H_(2n+1)) wherein n is forexample 2-18, as the hydrocarbon group R, and it has a multi-layerstructure wherein inorganic layers formed by octahedronal clusters of alead halide are separated by an organic material. However, in thisinvention, the volume of (R—NH₃) or (R′═NH₂) is less than the volume oflead halide clusters, so the inorganic layers are not separated by theorganic material, an inorganic three-dimensional network is formed, andthe organic material instead penetrates the gaps in the octahedronalclusters of metal halide. The basic structure at room temperature isshown in FIG. 1. FIG. 1 shows how the organic material represented by(R—NH₃) or (R′═NH₂), is occluded in the spaces between the metal (e.g.,lead) halide of this invention.

[0022] The detector of this invention comprises a scintillator and alight-receiving device, a three-dimensional perovskite compound beingused as the scintillator.

[0023] The three-dimensional perovskite compound used in this inventionis the compound represented by the general formula AMX₃, where A is isR—NH₃ or R′═NH₂, or a mixture thereof.

[0024] Herein, the conditions regarding (R—NH₃) or (R′═NH₂) are thatthey should be monovalent cations of such a size that they can beoccluded within the aforesaid three-dimensional compound. Specifically,R is methyl or hydrogen, and this methyl group may be substituted by anamino group or halogen atom. R′ represents a methylene group, and thismethylene group may be substituted by an amino group or halogen atom.Examples of this (R—NH₃) or (R′═NH₂) are H—NH₃, CH₃—NH₃ and NH₂CH═NH₂(formamidinium cation). However, in the case of C₂H₅—NH₃, the product isa two-dimensional stratified compound (Japanese Unexamined PatentApplication No.2001-006132), and not the three-dimensional compound ofthis invention.

[0025] X in the aforesaid general formula represents a halogen atom,preferably Cl, Br or I. From the viewpoint of stability of the compound,Br is most preferred, but from the viewpoint of low LET radiationdetection, I which has a large atomic number is most preferred. Also, Xmay be a mixture of these halogens. M is a Group IVa metal, Eu, Cd, Cu,Fe, Mn or Pd, preferably a Group IVa metal or Eu, more preferably aGroup IVa metal, still more preferably Ge, Sn or Pb, and most preferablyPb.

[0026] This scintillator is preferably a single crystal describedhereafter, but it is not necessarily a single crystal, and may be apolycrystal for example coated by spin coating or the like on a solidsubstrate. This solid substrate must not emit luminescence which wouldinterfere with measurements, therefore silicon crystals may for examplebe used.

[0027] As this scintillator emits light in the visible region, aphotomultiplier or the like may be used as the light-receiving device.Typical examples are a construction wherein the scintillator is incontact with the light-receiving surface of the photomultiplier, aconstruction wherein the scintillator and the photomultiplier areconnected by a light waveguide such as an optical fiber or the like, anda construction wherein the light emitted by the scintillator is receivedby a light-receiving port separated from the scintillator, thislight-receiving port being connected to the photomultiplier by a lightwaveguide. The signal from the light-receiving device is processed bythe usual method.

[0028] The scintillator in the radiation detector of this invention hasa high crystal-forming ability, and single crystals of large volume canbe formed. Therefore, the high-speed exciton luminescence can be appliednot only to the detection of high LET radiation beams such as α-rays andheavy charged particle beams, but also to the detection of low LETradiation beams such as γ-rays, X-rays and high-speed electron beams.Further, it may also easily be applied to the detection of short pulsesof low LET radiation beams which were difficult to detect in the priorart.

[0029] The radiation detector using the three-dimensional perovskitecompound of this invention, e.g., (CH₃NH₃)PbX₃, has the followingcharacteristics.

[0030] As the scintillator, i.e., the perovskite organic/inorganichybrid compound of this invention, has an increased luminescenceintensity the lower the temperature is, it is preferred to cool themeasurement system.

[0031] The scintillator is easily manufactured. When theorganic/inorganic hybrid compound deposits from an organic solution, athree-dimensional network of self-organizing lead halide clusters isformed, so it can be very economically mass-produced without requiringhigh temperature or high pressure as in the case or inorganic crystalsor ceramic scintillators.

[0032] As the exciton luminescence peak of the organic/inorganic hybridcompound is unique (e.g., in the case of (CH₃NH₃)PbBr₃, 550 nm), themeasurement system can be simply constructed from a light waveguide andlight-receiving device alone.

[0033] Hereafter, this invention will be described by means of specificembodiments, but the invention is not to be construed as being limitedin any way thereby.

EXAMPLE 1

[0034] 60.22 g hydrobromic acid (HBr, Wako Pure Chemicals, concentration0.48) was introduced in a 200 ml flask at room temperature, and 27.06 gof 40% aqueous methylamine solution (Wako Pure Chemicals, concentration0.41) was gradually dripped in. As this is an exothermic reaction, theflask was placed in a water bath. Methylamine was dripped until themolar ratio of hydrobromic acid, HBr, to methylamine, CH₃NH₂, was 1:1.After addition was complete, the mixture was left with stirring for 1hour to complete the reaction, and a colorless, transparent aqueoussolution of methylamine bromide was thus obtained. When water wasremoved on an evaporator (water bath temperature 45° C.), a white powderof methylamine bromide remained. This was washed by diethyl ether(suction filtration), and after removing unreacted material, it wasdried. The yield was 35.98 g, i.e., 90.0%.

[0035] Next, 18.8 g of the methylamine bromide obtained as mentionedabove was dissolved in 100 ml DMF in a 200 ml three-necked flask at roomtemperature, and 61.62 g lead bromide, PbBr₂ (Highly Pure Chemicals,purity 99.99%) was added a little at a time until the molar ratio ofmethylamine bromide and lead bromide, PbBr₂, was 1:1. To avoid reactionbetween the moisture in the air in the three-necked flask, the mixturewas left with stirring for 1 hour to complete the reaction whilesteadily passing a current of dry nitrogen through the flask, and a DMFsolution (transparent and colorless) of the perovskite type compound,(CH₃NH₃)PbBr₃, was thereby obtained. The solvent was evaporated on anevaporator (water bath temperature approx. 80° C.), and amicrocrystalline powder of a red perovskite compound remained. This waswashed by diethyl ether to remove unreacted material, and dried. Theyield was 78.41 g, i.e., 97.5%.

[0036] Next, a single crystal of the perovskite compound used as thescintillator was prepared by the poor solvent diffusion method using thedevice shown schematically in FIG. 2 (Reimei Hirayama, “Organic CrystalManufacturing Handbook”, Chapter 8, “The Crystallization of OrganometalComplexes”, 2001, Maruzen Publishing Co.).

[0037] The microcrystalline powder of the obtained perovskite compoundwas dissolved in as little of a good solvent (dehydrated DMF) aspossible, and undissolved material was filtered off using a filterhaving a retention capacity of about 0.1 micrometers (MILLIPORE,Millex-LG SLLGH₂5NB). This solution was introduced into a container(glass bottle A) for depositing crystals. Glass bottle A was subjectedto ultrasonic cleaning with pure water beforehand. Next, a poor solvent(toluene, diethyl ether, nitromethane, etc.) was introduced into a glassbottle B. In order to dehydrate the poor solvent, a little calciumchloride powder was also introduced into glass bottle B. Glass bottle Aand glass bottle B were stored in a desiccator, sealed off from theatmosphere, and left for four days at room temperature. At this time,the poor solvent which evaporated from glass bottle B spread into theperovskite compound solution in glass bottle A so that the solubility ofthe solution in glass bottle A gradually fell, and red, transparentsingle crystals of perovskite type compound deposited on the bottom ofglass bottle A. Glass bottle A was shaded by wrapping the wholedesiccator in aluminum foil. Of the single crystals thus obtained, thosewith the largest volume measured 1 cm×1 cm×5 cm.

[0038] When the obtained single crystals were excited using an electronbeam pulse of 200 femtoseconds accelerated to 30 MeV by a linearaccelerator (LINAC) in vacuo (approx. 10⁻⁶ torr), a luminescence with apeak wavelength of 550 nm was observed. The time transition ofluminescence intensity of this luminescence was measured using a streakcamera (Hamamatsu Photonics, Inc., FESCA-200) with a resolving time of260 femtoseconds as light-receiving device. This device is shownschematically in FIG. 3, and the result is shown in FIG. 4. As a resultof this numerical analysis, the decay time constant of this luminescencewas approx. 240 picoseconds.

EXAMPLE 2

[0039] While varying the temperature of the single crystals manufacturedin Example 1, a scintillation luminescence spectrum from the sample wasmeasured by irradiating it with hydrogen ions of 2.0 MeV using a Van derGraaf accelerator (Tokyo University Atomic Energy Research Center). Themeasurement result showed an identical relation to the relation betweenluminescence intensity due to irradiation with ultraviolet light (He—Cdlaser), and temperature.

[0040] The result of irradiation with ultraviolet light is shown in FIG.5. Taking the reference value of luminescence intensity for NaI(Tl) as100, the luminescence intensity of this compound at 300K was 0.075, andat 25K was 140. The luminescence intensity decreases exponentially as afunction of the absolute temperature.

EXAMPLE 3

[0041] In this example, the single crystal manufactured in Example 1 wasirradiated with γ-rays, and it was confirmed that this single crystalcould detect the γ-rays.

[0042] A schematic view of the system used in this test is shown in FIG.6. ²²Na was the sealed source of the γ-rays, and the intensity was 370Bq (Becquerels). The single crystal was sealed in a cryostat coldfinger, and cooled to 40K. The luminescence was directly received by aPMT (photomultiplier, Phillips, XP4222B) attached to a quartz glasswindow. The signal from the PMT was amplified by an AMP, and recorded asan energy spectrum by an MCA (wave height discrimination machine). Theresult is shown in FIG. 7.

[0043] In FIG. 7, the solid line shows the signal intensity when thesingle crystal was installed and cooled to 40K. On the other hand, theblack shaded part shows the noise level for the signal intensity whenthere is no scintillator crystal. From the difference, it can be seenthat the single crystal emits a scintillation luminescence when γ-raysare received.

What is claimed is:
 1. A radiation detector comprising as a scintillatoran organic/inorganic perovskite hybrid compound represented by thegeneral formula AMX₃, wherein A is R—NH₃ or R₁—NH₂, or a mixturethereof, R is a hydrogen atom or a methyl group which may be substitutedby an amino group or a halogen atom, R′ is a methylene group which maybe substituted by an amino group or a halogen atom, each X is a halogenatom that may be identical to or different from the other X groups, andM is a Group IVa metal, Eu, Cd, Cu, Fe, Mn or Pd.
 2. The radiationdetector according to claim 1, wherein the form of saidorganic/inorganic perovskite hybrid compound is a single crystal.
 3. Theradiation detector according to claim 1 or 2 which is adapted to detectlow LET radiation.
 4. The radiation detector according to claim 3,wherein said low LET radiation is a pulse.